Signal level indicator



Nov. 3, 1953 J. 5. HARRIS 2,658,167

SIGNAL LEVEL INDICATOR Filed Aug. 28, 1952 INVENTOR.

JAMES E. HARRIS 17 'ITORNE I Patented Nov. 3, 1953 2,658,167 SIGNAL LEVEL INDICATOR James S. Harris, 01d Greenwich, Conn, assignor to Radio Corporation of America, a corporation of Delaware Application August 28, 1952, Serial N 0. 306,880

10 Claims.

This invention relates broadly to signal level indicators, and more specifically to a signal level indicator by means of which a signal voltage may be compared with a reference voltage of predetermined value. While not specifically limited thereto, the signal level indicator of the present invention is particularly useful for indicating the signal level of a tone signal converter, the output of a tone keyer, or the aggregate signal level on a communication channel carrying a signal such as a tone.

The usual method of comparing a signal voltage with a fixed reference voltage in a system is to connect a meter, of the moving coil type, between the two points in the system representative of the signal voltage and the reference voltage, respectively. In addition to the expense of this meter and the difiiculty of determining the exact point of balance in the usual case, an objection to such instruments is that there are at least three conditions which may lead an observer to the erroneous conclusion that the signa1 voltage is equal to the reference voltage. For instance, if any of the tubes fail or any connections in the circuit are open the meter will read zero. Also, a failure of the power supply voltage, or meter burn out, will result in a zero reading on the meter.

In the invention to be described, small neon or other gas type lamps are used as indicators. These have the advantage of being inexpensive and, when properly protected against excessive current, have a, relatively long life. Neon lamps, for instance, are not normally subject to sudden failure but indicate the need for replacement by a gradual darkening rather than by a complete burn out.

It is accordingly a principal object of the present invention to provide an improved signal level indicator.

It is a further object of the present invention to provide an improved signal level indicator,

employing gas type lamps, which is relatively failsafe as compared to circuits using a moving coil type meter.

Another object of the present invention is to provide an improved signal level indicator using a circuit which compensates for any non-uniformity of the gas type lamps employed.

Still another object of the present invention is to provide an improved, inexpensive and reasonably accurate signal level indicator which is simple in construction and operation, which renders protection against erroneous interpretation, and which is highly efficient in use.

' be lighted, thus indicating According to the invention, these and other objects and advantages are attained in an improved signal level indicator comprising a differential amplifier circuit using a pair of electron tubes operating under conditions of substantially constant current. The cathodes of the tubes are connected to a common impedance and in the anode circuit of each tube is a neon or gas type lamp. A source of unidirectional potential is applied across the anode-cathode circuit of each tube. The grid of one tube is supplied with a fixed reference potential. The signal voltage, whose level is to be determined, is applied to the grid of the other tube. If the level of the signal voltage is within anadjusted db (decibel) range of the reference voltage both neon lamps will be lighted. If the level or the signal voltage is above or below the adjusted db range of the reference voltage, only one or the other neon lamp will whether the signal is above or below the desired, adjusted db range.

Gas type lamps usually exhibit considerable non-uniformity between samples of the same type. This is especially true with respect to the firing potential of these lamps, and previous circuits using these lamps were considered unsatisfactory because of their inherent non-uniformity. In accordance with the circuit of the present invention, the effects of such non-uniformity are minimized and will normally be inconsequential in the contemplated usages of the circuits.

For a more detailed understanding of the invention reference is made to the accompanying drawings, in which similar reference characters are applied to similar elements, and in which:

Fig. 1 is a schematic drawing of a signal level indicator in accordance with the present invention, 4

Fig. 2 is a schematic diagram of a modified form of the signal level indicator of Fig. 1,

Fig. 3 is a schematic diagram of another modification of the signal level indicator of Fig. 1.

Fig. 4 is a schematic diagram of a modification of the signal level indicator of Fig. 3, and

Fig. 5 is a schematic diagram of a further modification of the signal level indicator of Fig. 1.

Referring more particularly to Fig. 1, there is shown the signal level indicator in accordance with the present invention comprising a differential amplifier having a pair of electron discharge tubes IG and I2. The cathodes of the tubes iii, [2 are connected to the negative terminal of a source of unidirectional voltage l4 through a common biasing resistor [6. The resistor i6 is a common impedance for the cathode circuits of the tubes l0, l2. Connected between the positive terminal of the voltage source l4 and the anode of the tube I is a current responsive device such as a gas type lamp IS. A lamp 20, similar to the lamp I8, is connected between the positive terminal of the voltage source l4 and the anode, of. thev tube l2. A reference voltage of suitable value may be applied to; the grid of the tube I'Z'by connecting the latter to a suitable voltage of the voltage source l4 through a variable tap 22. The signal, whose voltage level is" to be determined, is applied to input: terminals 24, 26. The signal, applied to the grid of the tube In through a current limiting resistor 23, must be a D.-C. voltage. A. C. signals'maybe handled if they are rectified before they are applied to the grid, in a manner-we'll: knnwriinthe electronic art.

If the potential difference between the negative" terminal of the voltage source l4 and the reference voltage is fairly large, that is several times as great as the" negative control potentialre-- quired to cut off the anode current in either of the tubes H) or [2, and if the resistance [6 and the impedances of the lamps i8", are of suit able values, considering and the reference voltage applied to the grid of the tube l2, it will be found that the amplitudeof the signal voltage applied to the grid of the tube It! will be the major factor in the determination of the values of anode current passed between tubes 0, [2. Under these conditions the difference in the-impedances between lamps l8 and 26 caused byany non-uniformitytherebetween, will have littleor no effect on the values of the anode current. Within these limitations,

it has been observed. that for any given signal and reference voltages the tubes [0, l2 function as constant current tubes and that neither the plate load resistance nor the hand-book values of the tube plate resistance areof any great consequence. Under'theseconditions, thetubes I0, I 2 compensate for any-change in plate loadresist ance by an opposite change in plate resistance, thus maintaining a constant totalresistance-be tween the tube cathodes and the-- positive ter minal of the voltage source M. It will be understood that the division of' current between the anode circuits of thetub'es HF and I2 is determined wholly by the relativepotential difference between the signal and reference-voltages applied to the tubes 50- and" If; respectively.

The operation of the signal level indicator; as shown in Fig; 1, will now be explained. When the signalvoltage applied to the grid of the tube l0 equals the referencevoltage applied to the tube l2, the anode currents iii eachtube will be equal, and both of the lamps l8 and 20 will be lighted. If the sign l voltage-issufiicientl-y negative with respect tothereference voltage, the tube l0 will ceaseto pass current while the tube l2 will conduct an increased current therethrough, and the lamp Zll'will glow with increased intensity while the lamp ['8 will be extinguished. If the signal voltage'i's sufficientlypositive with respect to the reference voltage, then the lamp 20 will be extinguished and the lamp lBwill glow brightly. It will be-noted that, regardless of thelevel of the signal voltage applied to the tube In, either one or'both of theneon lamps will be lighted. In this respect, the signal level indicator; in accordance with the present invention, is fail-safe. If a meter ofthe moving coil type were to be hooked across the anodes of the tubes Ill, l2 in order to indicate the difference in voltthe supply voltage l4" age at these points, the meter would read zero if the signal voltage applied to the tube l0 were equal to the reference voltage applied to the tube [2 assuming that the anode impedances of the tubes I0, I2 are equal. A zero reading on the meter, however, would also be caused by a failure of either of the tubes l0, l2, a failure of the power supply M, ore. burn out imthe meter itself. Under the latter conditions, an operator reading the meter could never be absolutely sure that the signal level indicator circuit is functioning properly; In accordance with the present invention, when the signal voltage is equal to the reference voltage; both of" the lamps l8 and 20 are lighted, thus indicating aproper functioning of the signal level indicator. If both of the lamps l8 and 20 were-extinguished they would definitely indicate a fault in the signal level indicator since under noconditions of the level of the signal voltage should both of the lamps l8 and 20 be extinguished.

It was previouslyv mentioned that the gas type lamps IB-and 20 may be somewhat non-uniform in characteristics. As-explained above, however; the circuit as shown in Fig. 1 has the effect of determining the current flow through the lamps I8, 20: without regard to their uniformity and thus the ignition and extinction voltage characteristics of these lamps lose their significance in determining the accuracy of the circuit. Therefore, quite definite limits can be determined for the requiredpotential differences between the signal and the reference voltages to just ignite or just' extinguish either of the two lamps, and

if both lamps are lighted th'e potential differencesare immediately known tube within established limits: In many instances; the indication of 'both lamps lighted will be sufficiently accurate for the requirement; However, at either of the limits, it is obvious that one lamp will be at practically maximum intensity while the other will be at much less intensity; and that a closer balancebetween the signal the reference voltage will produce acondition of equal intensity of the two lamps. Fortunately; the pointer equal intensity, as observed by the eye, represents approximately equal current in twolampsof the same type so that an adjustment that produces equal intensity is a close indication ofbalance between-the signal and the reference voltage;

It is evident that, for a given gas tube type, the potential difference required to reach the two extinction points is determined by the-tube design characteristics, and is relatively constant over'awiderange ofabsolute values of signal and reference-voltages. Thusthe larger the absolute values, the smaller the percentage betweenthe two voltages will be for the condition under which both lampswill be lighted. If the. absolute values are fixed for a given requirement, the higher the amplification factor of the tubes used, the smaller willbe the allowablediiference between thesi'gnal and'the reference voltages for both lamps to be lighted. Thus, in actual practical circuits, the designer can design the circuit to practically any accuracy required. A further advantage which accrues from a circuit of this type is the fact that it can utilize triode tubes operating as essentially constant current tubes. Under these conditions, the tubes service life islengthened and the transconductance can drop considerably before the circuit is appreciably affected. While triodes may be preferable in the use of these circuits, multi-element' tubes may also be used in a manner well known in the art.

In certain instances, as when the signal voltage applied to the grid of the tube I0 is excessively large, the characteristic of the circuit shown in Fig. 1 that produces a maximum current in the tube ill will be undesirable. If the signal voltage is excessive, the grid of the tube I 0 tends to draw more current than is desired. In such cases, the cathode resistance 56 may be replaced by an electron tube functioning as a constant current device, as shown in Fig. 2. With reference to Fig. 2, there is shown a signal level indicator similar to the signal level indicator of Fig. 1 except that the common cathode circuit of the tubes ill, I2 comprises an electron tube having its anode connected to th cathodes of the tube I 52, and its cathode connected to the negative terminal of the voltage supply M through a cathode resistor 32. The grid of the tube at is connected to a source of suitable unidirectional voltage supply I 6 through a tap 3 3. With the circuit designed for a certain reference voltage, the tap may remain fixed. The operation of the signal level indicator shown in Fig. 2 is the same as that described for the circuit of Fig, 1 except for the fact that an excessive signal voltage applied to the grid of the tube I s will not cause an excessive amount of current flow in the anode cathode circuit of the tube I ii. The excessive current is prevented because of the degeneration resulting from the cathode resistor 32 in 9 the cathode circuit of the tube Bil. Under these conditions, the tube L l! acts as a constant current tube.

In many cases, it will be desired to extend the limits between the extinguishing potential difiorences required for the two lamps it, 26. Figs. 3 and a show means which accomplish this result. In Fig. 3, there is shown a signal level indicator similar to Fig. 1 except for the fact that the common cathode impedance between the cathodes of the tubes Is, it? comprises a 1r network. The cathodes of the tubes 59, I2 are connected to each other through a resistor 36. The cathodes of tubes ill and 42 are also connected to the negative terminal of the voltage source It, through resistors 38 and 48, respectively. A voltage divider comprising resistors 42, 44 and 46, in series with each other, is connected across the voltage supply M. The reference voltag supplied to the tube I2 is taken from a selected point on a resistor i4 by means of th variabl tap 22. In operation, the 1r coupling network, comprising resistors 35, 38 and it, maintains relative sensitivity between the signal voltage and the ref erence voltage for diiferen't settings of the reference voltage. If the requirement is such that signal voltage shall match a fixed reference voltage within a specified diiference, the resistors 36, 38 and t may b If, however, it is desired to determine the value of the signal voltage within a fixed percentage difference of an adjustable reference voltage, the resistor 36 may be a variable resistance which may be ganged to the variable tap 22 f0r movement therewith, as shown in Fig. 3.

The signal level indicator circuit of Fig. 4 is substantially the same as that shown in Fig. 3 except that the 1r network shown in Fig. 3 has been replaced by a T network. In Fig. 4, the cathodes of the tubes I a, I2 are connected to each other through resistors 28 and 50, in series with each other. The common junction between the resistors 48 and 58 is connected to the negative terminal of the voltage supply I4 through a resistor 52. The operation of the circuit of Fig. 4

6 is similar to that of Fig. 3. If it is desired to maintain the same sensitivity between the signal voltage applied to the tube Ill with respect to the reference voltage applied to the tube I2, for all levels of the latter reference voltage, the resistors 48 and 56 may be variable and ganged to the variable tap 22 for movement therewith.

If it is desired to increase the sensitivity of the signal level indicator, regeneration may be used to reduce the allowable potential differences required to extinguish one of the lamps. Fig. 5 illustrates such a modification. The signal level indicator of Fig. 5 is similar to the circuit shown in Fig. 1 except for the fact that a load resistor 54 i inserted between the positive terminal of the voltage source IQ and the lamp I 8. A resistor 5c is connected to the junction between the resistor 54 and the lamp I8 and the negative terminal of the voltage sourc I l. The reference voltage applied to the tube i2 is taken from a selected point on the resistor 56 by means of the variable tap 22. The sensitivity of this circuit is increased by the feedback from th anod circuit of the tube I6 to the grid of the tube I2. In operation, if the signal voltage applied to the grid of the tube I I3 is greater than the reference voltage applied to the grid of the tube I2, a negative going voltage will be fed back from the junction between the resistor E l and the tube I8 to the grid of the tube I2 and thereby further increase the diiierence between the signal voltage of the reference voltage. It is also understood that if the signal voltage applied to the grid of the tube IE is smaller than the reference voltage applied to the grid of the tube I 2, the fed back signal to the grid of the tube I2 will be positive going, thereby increasing further the potential differences between the signal voltage and the reference voltage. The diiferential type amplifier having constant current characteristics is particularly suitable for the application of regeneration in that the gain may be greatly increased while maintaining good stability. Although the circuit arrangements described and illustrated are especially suitable for use with gas type lamps, filament type lamps may be substituted when desired provided adequate current can be accommodated by the tubes. For normal use, the greater efficiency and freedom from sudden burn out of the gas type lamps make them preferable.

Thus, it is seen that there has been provided, in accordance with the objects of the present invention, a signal level indicator comprising two electron discharge tubes having a, common cathode circuit and a current responsive device individually connected in their anode circuits. When stantially equal, within a desired db range, to the reference voltage applied to the grid of the other tube, both lamps will be lighted. If the signal voltage is greater or smaller than the reference voltage, one or the other of the lamps will be lighted. Under no conditions of normal operation will both lamps be extinguished. Several embodiments have been shown and described for varying the sensitivity of the signal level indicator by varying the ratio of the signal voltage to the reference voltage. An arrangement also has been shown and described whereby an excessive amount of grid current is prevented from fiowing through the tube to which the signal voltage is applied by employing an electron tube in the cathode circuit and employing a cathode resistor for producing degeneration.

While the signal level indicator of the present acsazcv invention. has been described. in connection with a; D=.-C. signal voltage; it is evident; to those skilled. in the art that A.-C. signals may be handled by using rectifier circuits, and pulsed signals may be used by employing. peak. reading circuits.

What is claimed is:

15. A signal level. indicator circuit comprising a pair of electron discharge tubes each having at least a cathode, a control electrode, and an anode, neonlamps individually connected to each of said anodes, a common impedance connected to each of. said cathodes, means to apply a source of. unidirectional. voltage across" said lamps and said impedance, means to apply a reference voltage to. a control electrode of one of. said tubes, and. means toapply a signal voltage to the control electrode of the other of said tubes.

2. A- signal level indicator circuit as defined in claim I wherein said common impedance comprises an electron discharge tube, and means to bias said. last-mentioned tube as a constant current tube.

3'. A signal level indicator circuit as defined in claim 1 wherein. said common impedance comprises an electron discharge tube having at least ananode, a control electrode, and a cathode, a resistor. connected. between said last-mentioned cathode and said voltage applying means, said last-mentioned anode being connected to the cathodes of said pair of. electron discharge tubes, and. means to apply a voltage to said last-mentioned control electrode.

4. A. signal level indicator circuit as defined in claim. 1 wherein said common impedance com.- prises a resistor connected betweensaidcathodes, and resistors connected individually between each cathode and said voltage applying means.

5. A signal level indicator circuit as defined. in claim 1 wherein said common impedance comprises a first resistor, a second resistor connected in series therewith, said first and second resistors being connected between said cathodes, and a third. resistor connected between the junction. of. said. first and second resistors and said voltage applying means.

6. A signal level indicator circuitcomprising a pair. of electron discharge tubes each having a. grid, an. anode circuit, and av common cathode circni a separate lamp in the anode circuit of each tube, means. to; the grid of one of said tubes, means in series with said anode and cathode circuits to apply a source of. unidirectional voltage thereacross, and means to apply a unidirectional signal voltage to the grid of the other of said tubes, whereby the flow of current ineach anode circuit is controlled in response to the magnitude of said signal voltage.

'7. A. signallevel indicator circuit as defined in claim 6 wherein said common cathode circuit comprises a third electron tube having at least an anode, a control electrode, and a cathode, said lastementioned anode being connected to said cathodes of said pair of. tubes, and means connected, between. said last-mentioned cathode and said last-mentioned control electrode to bias said last-mentioned control. electrode.

8. A signal level indicator circuit as defined in claim 6 wherein said. common cathode circuit comprises a first resistor connected between said cathodes, and a second and third resistor each connected between a cathode respectively and said meansto apply a source of unidirectional voltage.

9. A signal, level indicator circuit as defined in claim 6 wherein said common cathode circuit comprises a T-network. of resistors. having a first end connected to a cathode of one tube, a second end of said T-network connected to a cathode of the other. of said tubes, and a third end of said T-network connected. to said means to apply a source of unidirectional voltage.

10. A signallevel indicator circuit as defined. in claimv 6. wherein the anode circuit of said other of said tubes comprises a first impedance between said lamp and. said means to apply a unidirectional voltage, said means to apply a reference voltage comprises a second end connected to the junction of said last-mentioned lamp and. said first impedance and the other end connected to said means to apply a source of unidirectional. voltage, and means to connect. the. grid of said one of. said tubes selectively to. a point on said second impedance.

JAMES S. HARRIS.

No references cited.

to; apply a reference voltage.

impedance having one- 

